Fill interactive help gaps for all new UI elements
Added ~30 HELPER_CONTENT entries covering:
- Sidebar: Downloads page, Playlist Explorer nav buttons
- Dashboard: all 10 enrichment service worker pills, recent syncs,
API rate monitor, maintenance, SoulID, blacklist
- Sync: Server Playlists tab, ListenBrainz tab
- Active Downloads page: filters, list, clear button
- Playlist Explorer page: picker, view toggle, action bar
- Issues page: header, filters, findings list
- Discover: Your Artists carousel
- Personal Settings gear button
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